This invention relates to the use of electronic display materials for electric reusable paper applications. The invention is designed for use with Gyricon electric reusable paper but may also be used with electric reusable paper based on liquid crystal, electrophoretic, and other field-effect display technologies.
Electric reusable paper can be defined as any electronically-addressable display medium that approximates paper in form and function. Electric reusable paper should be light-weight, thin, and flexible, and it should display images indefinitely while consuming little or no power. In addition, electric reusable paper should be re-usable. One must be able to erase images and create new ones repeatedly. Preferably, electric reusable paper should display images using reflected light and allow a very wide-viewing angle.
One way to make electric reusable paper possible using traditional electronic display technology is to completely remove the driving electronics from an electronic display package and use external addressing electrodes to write and erase images. This approach both reduces the per unit cost of electronic paper sheets and enables the use of cheap, flexible plastic films in place of glass plates for packaging. Multiple electronic paper sheets can then be addressed by a single set of external driving electronics, much like multiple sheets of pulp paper are printed on by a single printer.
A sheet and display system dubbed Gyricon is disclosed in various patents and articles, such as U.S. Pat. No. 4,126,854 by Sheridon titled xe2x80x9cTwisting Ball Displayxe2x80x9d. The Gyricon display system is comprised of a host layer a few mils thick which is heavily loaded with bichromal elements, possibly spheres, tens of microns in diameter. Each bichromal element has halves of contrasting colors, such as a white half and a black half. Each bichromal element also possesses an electric dipole, orthogonal to the plane that divides the two colored halves. Each bichromal element is contained in its own cavity filled with a dielectric liquid. Upon application of an electric field between electrodes located on opposite surfaces of the host layer, the bichromal elements will rotate depending on the polarity of the field, presenting one or the other colored half to an observer.
An Electric reusable paper substrate has many of the requisite characteristics of electric reusable paper, namely, bistable image retention, wide viewing angle, thin and flexible packaging, and high reflectance and resolution. U.S. Pat. No. 5,389,945 issued to Sheridon on Feb. 14, 1995, and titled xe2x80x9cWriting System Including Paper-Like Digitally Addressed Media And Addressing Device Thereforxe2x80x9d, describes an electric reusable paper printing system that employs independent, external addressing means to put images on the Electric reusable paper substrates. The external addressing means is described as a one-dimensional array of electrodes connected, either directly or by wireless technology, to modulating electronics. As the one-dimensional array is scanned across the sheet, modulating electronics adjust the potential at the individual electrodes, creating electric fields between the electrodes and an equipotential surface. An image is created in the sheet according to the polarity of the electric fields. However, fringing fields in the vicinity of the addressing electrodes can cause incomplete or excessive rotation of the imaging elements in the sheet. Optical properties of improperly rotated imaging elements are not optimal because of their over-rotated orientations.
A second issue is the return-to-zero effect, that limits the ability to address Electric reusable paper substrates with external addressing. Positive and negative mobile ionic charges are present in the sheet as well. In regions of the sheet trailing the path of the external addressing device, and no longer under the influence of an applied external electric field, mobile ionic space charges create an electric field opposite to the previously applied field which imparts torque on the bichromal elements contained therein. This torque can dislodge the bichromal element 220 from its intended position, determined by the external addressing device, leaving it in an optically-poor position for viewing.
A third issue facing electric reusable paper printing systems is that sheets, once printed on by some external addressing device, are subject to inadvertent tribo-electric writing. In the described electric reusable paper printing system, images are produced willfully by an external addressing device that has the ability to create electric fields. Electric charge applied inadvertently by tribo-electric exchanges during handling can equally create electric fields that cause image change. This effect poses a threat to image retention and stability. It should be emphasized that this significant problem is a threat to any electric reusable paper technology which uses field-addressed electric reusable paper sheets including Gyricon, liquid crystal and electrophoretic technologies.
A fourth issue facing the use of external addressing devices on electric reusable paper sheets is that one-dimensional external addressing devices are limited in how quickly they can print an image on an entire sheet by the response speed of optical display elements. In Electric reusable paper substrates, complete rotation of bichromal elements is only achieved if the addressing electric field is held at least as long as the required rotation time, on the order of 100 milliseconds. For a sheet on which many rows of an image must be printed it would take many seconds or even possibly minutes to display an entire image.
A final issue facing electric reusable paper is the difficulty of producing color versions. U.S. Pat. No. 5,717,515 by Sheridon issued Feb. 10, 1998 and titled xe2x80x9cCanted Electric Fields For Addressing A Twisting Ball Displayxe2x80x9d describes several methods for making highlight color and full color versions of a electric reusable paper substrate and display. These systems all require multi-segmented spheres instead of bichromal spheres. That is, the rotational elements needed to implement a color system have at least three different segments instead of the two segments used in the bichromal spheres. While production of multi-segmented spheres is possible the fabrication techniques needed are more complex and therefor the multi-segmented spheres are more difficult to manufacture than bichromal spheres. Additionally, these implementations use the complex addressing techniques of canted fields, multithreshold multipass addressing or addressing requiring multiple electrode addressing layers. Canted field addressing requires the generation of electric fields that are not substantially perpendicular to the viewing surface while multithreshold multipass addressing requires the usage of spherical elements which rotate when different strengths of electric fields are applied. In short, all of these systems are more complex and more difficult to implement than typical gyricons using bichromal rotational elements.
One proposed solution to the above problems has been to use charge retaining island patterning on the electric reusable paper sheets. This technique has been described in copending U.S. patent application Ser. No. 09/037,767 by Howard et al., titled xe2x80x9cCharge Retention Islands For Electric Paper And Applications Thereofxe2x80x9d, incorporated by reference hereinabove.
In summary, charge retaining island patterning is an electric reusable paper sheet that uses a pattern of conductive charge-retaining islands on the outward-facing side of at least one of two opposed outward surfaces. The second outward surface may also be coated with a conductive material, or made of a conductive material, and may or may not be patterned. The charge-retaining islands of the patterned surface or surfaces receive electric charges from an external charge-transfer device. The external charge-transfer device could have a plate configuration and be held over and in contact with the sheet, it could have a wand configuration and be pulled across the sheet, or it could have a stylus configuration and be used like a pen or pencil. After the charge-transfer device is removed, the conductive, charge-retaining islands hold electric charge, creating an electric field in the electric reusable paper of sufficient magnitude and duration to cause an image change.
Unfortunately, as useful as the charge retaining island approach has been in mitigating the above problems, it can introduce a new image stability issue because the charge retaining islands retain charge for a some period of time after they have been addressed and the image has changed in the electric reusable paper sheet. This instability arises because fully charged islands on an electric reusable paper sheet can easily have charge removed by contact with a grounded device, such as a person""s finger. Rapid charge removal of the charge on the charge retaining islands, such as when immediately handled subsequent to addressing, can cause some rotational elements to rotate and create image defects such as streaks or fingerprints, due to the return-to-zero effects outlined earlier.
However, an alternate embodiment of the charge retaining island approach does not suffer from the above instability. The alternate embodiment utilizes charge retaining islands which are created as part of the bulk of the encapsulating layer instead of being patterned on the surface of the layer. Extending the conductivity of the charge retaining islands through the bulk of the encapsulating layer to the sheet contained therein improves the performance of the charge retaining islands and reduces the problem of image instability when handled immediately after addressing. This embodiment is implemented using a z-axis only conductive sheet material. Z-axis only sheet materials are generally made from an insulating host material which has been doped with conductive particles that transmit charge only in one axis. While this approach has been somewhat successful, it can suffer from the defect that constructing charge retaining islands with z-axis properties of any significant size is difficult. If the islands are too small and are too widely placed, not enough electric field is generated to accomplish element rotation and reliably addressing the charge retaining islands becomes difficult. Furthermore, construction of charge retaining islands of sufficient size may result in image degradation of the sheet because the conductive material used to dope the sheet may degrade the optical properties of the sheet.
An electric reusable paper substrate is provided with a charge retaining island structure on at least one surface. The charge retaining island structure is formed on a layer comprised of conductive portions separated by non-conductive portions. An island should be in contact with at least one conductive portion, however conductive portions should not be in contact with more than one island.